A polyacetal resin (also referred to as polyoxymethylene resin abbreviated to POM resin) has well-balanced mechanical properties and is excellent in, for example, abrasion and wear resistance characteristics, chemical resistance, heat resistance, and electrical characteristics, and is therefore widely used in the fields of, for example, motor vehicles and electric and electronic products.
Incidentally, the required characteristics in such fields are gradually becoming sophisticated. In order to meet such requirements, a polyacetal resin composition containing a polyacetal resin and a carboxyl group-containing compound having a pKa of 3.6 or more has been proposed (for example, see Patent Document 1).
The carboxyl group-containing compound is not particularly limited as long as the pKa is 3.6 or more, and examples thereof include various compounds having free carboxyl groups, such as, aliphatic carboxylic acids, alicyclic carboxylic acids, and aromatic carboxylic acids. Aliphatic, alicyclic, and aromatic polycarboxylic acids can also be used in a form having at least one carboxyl group, such as in dicarboxylic acid monoesters (e.g., monoethyl maleate, monomethyl fumarate, and monoethyl fumarate), tricarboxylic acid mono- or diesters, and tetracarboxylic acid mono- di-, or triesters. Preferred carboxylic acids are aliphatic mono- or dicarboxylic acids and aromatic monocarboxylic acids.
The polyacetal resin composition may further include, for example, an antioxidant, an alkali or alkaline earth metal compound, and a stabilizer. Preferred examples of the antioxidant include phenolic antioxidants (particularly, hindered phenols).
It has been proposed that at least one selected from polyalkylene glycols, fatty acid esters, fatty acid amides, fatty acid metal salts, etc. can be used as a processing stabilizer. As the fatty acid metal salt, a salt of fatty acid having 10 or more carbons and a metal can be used. The metal is preferably mono- to tetra-valent (particularly, mono- or di-valent) metal. Usually, an alkaline earth metal (such as Mg or Ca) salt is preferably used. Examples of the fatty acid metal salt include magnesium stearate, calcium stearate, zinc stearate, and calcium 12-hydroxystearate.
The polyacetal resin composition described in Patent Document 1 has an effect of greatly improving the thermal stability of the polyacetal resin (particularly, melt stability during the molding process). The composition also can suppress the generation amount of formaldehyde to an extremely low level and shows an effect of greatly improving the working environment. Furthermore, the composition can suppress the generation of formaldehyde even under severe conditions and can prevent decomposed products from adhering to the mold (mold deposit), leaching of decomposed products from the molded product, and thermal deterioration of the molded product to show an effect of improving the quality of the molded product and formability.
In addition, it is known that addition of a fatty acid metal salt as a component of the polyacetal resin composition contributes to improvement in physical properties of the welded part of a polyacetal resin molded product (for example, see Patent Document 2). The raw material fatty acid of the fatty acid metal salt is preferably myristic acid, palmitic acid, or stearic acid. The metal compound as the raw material of the fatty acid metal salt is preferably a hydroxide, oxide, or chloride of calcium. Preferred examples of the fatty acid metal salt include calcium dimyristate, calcium dipalmitate, calcium distearate, calcium (myristate-palmitate), calcium (myristate-stearate), and calcium (palmitate-stearate). In particular, the fatty acid metal salt is preferably calcium dipalmitate or calcium distearate.
In production of a polyacetal copolymer using trioxane as a main monomer (a) and a cyclic ether and/or cyclic formal having at least one carbon-carbon bond as a comonomer (b), it is also proposed to perform copolymerization using heteropoly acid represented by Formula (1) as a polymerization catalyst (c) and deactivation of the polymerization catalyst (c) by adding a compound (d) which is a carbonate, hydrogen carbonate, or carboxylate of an alkali metal element or an alkaline earth metal element or a hydrate thereof to the reaction product and melt-kneading the mixture (see Patent Document 3).Hm[M1x.M2yOZ].nH2O  (1)where, M1 represents a central element composed of one or two elements selected from P and Si; M2 represents one or more coordination elements selected from W, Mo, and V; and x represents an integer of 1 or more and 10 or less, y represents an integer of 6 or more and 40 or less, z represents an integer of 10 or more and 100 or less, m represents an integer of 1 or more, and n represents an integer of 0 or more and 50 or less.
It is also proposed to use a hydroxide or alkoxide compound of an alkali metal or an alkaline earth metal as a deactivating agent (d) of the polymerization catalyst (c) (see Patent Document 4).
According to the inventions of Patent Documents 3 and 4, not only a polymerization catalyst can be effectively deactivated but also unstable terminal portions can be stabilized by using a hydroxide or alkoxide compound of an alkali metal or an alkaline earth metal as a deactivating agent of the polymerization catalyst, and a high-quality polyacetal copolymer having excellent thermal stability and generating a very small amount of formaldehyde can be produced inexpensively through a simple manufacturing process.
In addition, compared to the conventional wet deactivation, the dry deactivation can rapidly and completely deactivate a polymerization catalyst and then stabilize unstable terminal portions through a very rationalized process in which a deactivation process is simplified and a washing process is omitted. As a result, a high-quality polyacetal copolymer that is thermally stable and has extremely small amounts of unstable terminal portions and formaldehyde emission can be produced inexpensively without problems, such as decomposition and degeneration, due to a catalyst.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2000-239484
Patent Document 2: Japanese Unexamined Patent Application, Publication No. H11-323076
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2014-105278
Patent Document 4: Japanese Unexamined Patent Application, Publication No. 2014-105279